Experiments will be carried out to examine changes in neuronal activity in the caudate-putamen and substantia nigra of the rat brain following intraperitoneal or intravenous administration of amphetamine. Changes in neuronal activity following saline or amphetamine pretreatment for periods ranging from 1 to 4 weeks will also be examined in an effort to determine if the effects of amphetamine are altered by long-term administration of the drug, consistent with the alterations in behavioral responses that occur following such pretreatment. The statistical analysis of spontaneous neuronal spike trains will be utilized to assess both the firing patterns of individual neurons in the caudate-putamen (autocorrelation analysis) and potential interactions between different classes of spontaneously active neostriatal elements (cross-correlation analysis). In addition to these studies, the morphology of the substantia nigra will be examined in serial sections under the electron microscope. Of special interest will be the existence of presynaptic dendrites identified in serial sections.